Commercially available plastics have been found to be useful because of the high rigidity of objects molded therefrom. However, this rigidity is often accompanied by brittleness or lack of toughness. Several blends of stiff polymers with other polymers have been made to improve the toughness of stiff polymers. One set of these blends involves mixing a stiff polymer with a soft, often rubbery polymer that is miscible with the stiff polymer on a molecular level. This produces a plasticized material that is essentially in one phase. However, this type of blend is almost always designed to produce flexible materials having reduced stiffness and lower heat resistance. Another set of polymer blends is prepared by combining a stiff polymer with certain polymers that are immiscible with it, creating a phase structure. The stiff polymer is typically a continuous phase, throughout which a soft polymer is located in a dispersed phase. A continuum of thermoplastic polymers with varying degrees of toughness is developed, depending on the amount of stiff polymer and soft polymer present. Another blend of polymers designed to increase toughness (such as already toughened ABS) is prepared by recycling degraded ABS and blending the recycled product with higher impact ABS. However, degraded ABS is often oxidized and crosslinked, and exhibits low flow which is detrimental to processing operations. Furthermore, ABS in general has poor weatherability characteristics (it ages and becomes embrittled at room temperature) and it thermally degrades due to processing heat. Blending in more ABS with the degraded ABS does not correct these deficiencies. In contrast, in the present invention a terpolymer not containing polybutadiene can be mixed with degraded ABS. Because a polymer with good weatherability has been added rather than the easily degradable polybutadiene, a longer lasting, toughened ABS may be formed.
U.S. Pat. No. 3,780,140 to Hammer discloses a polymer of ethylene, carbon monoxide and one or more monomers to produce solid products. The resulting polymer may be blended with solid organic polymers to produce flexible films and articles of varying flexibility. The patent recites a long list of solid organic polymers that can be used to produce a blend, including ABS resins. However, the ethylene/carbon monoxide/termonomer polymer must be compatible with the solid organic polymer. That is, the two polymers must preferably be miscible on a molecular scale. This suggests a one phase composition that is plasticized. In addition, the focus of this reference is to lower the modulus of the stiff polymer to varying degrees. In contrast, the blend of the present invention is directed to a toughened ABS resin that has a minimal reduction in stiffness and maintains a high modulus.
U.S. Pat. No. 4,613,533 to Loomis et al. describes a partially crosslinked, thermoplastic, elastomeric composition based on compatible blends of an ethylene/ester/carbon monoxide polymer and a vinyl or vinylidene halide polymer. There is no teaching of a composition including ABS resin.
U.S. Pat. No. 4,172,589 to Epstein discloses a toughened multi-phase thermoplastic composition in which one phase includes a polyester matrix resin and polycarbonate matrix resin and at least one other phase includes random copolymers. However, there is no teaching of a toughened ABS resin.
It is an object of the present invention to provide a soft, rubbery terpolymer that acts as an additional toughening agent for ABS resin. It is a further object of this invention to provide a toughened thermoplastic that retains its stiffness. It is yet another object of this invention to develop a toughened thermoplastic that retains a substantial percentage of both the original modulus and the heat distortion temperature (HDT) in a multi-phase polymeric system. These and other objects, features and advantages of the invention will become apparent in the description of the invention that appears below.